The present invention relates to a zoom lens unit, and a camera using the zoom lens unit as an imaging optical system for still images and/or for videos.
In recent years, a market of imaging devices (e.g., cameras) used for a security usage, etc. has grown, and requests or/and requirements from users are being various and diversified. Especially, most of the users demand a camera with higher image quality (resolution) and high magnification ratio, and it is highly important to achieve both of the high image resolution (high image resolving power) and high magnification ratio for zoom lens units used for photographing lenses.
To achieve the high image resolution, the zoom lens unit should have a resolving power compatible with the image pickup element of at least 1,000,000 to 5,000,000 pixels in the entire zooming range. Further, for the high magnification ratio, the zoom lens unit should have a magnification ratio of approximately 20 times.
Additionally, it is desirable that aberrations of the lens are sufficiently corrected even in the near-infrared light range, such that it enables to take an image under a dark environment.
Furthermore, it is also desirable to provide a zoom lens unit with a large diameter having an F number equal to or smaller than 2.0 at the short focal length end (also called “wide-angle end”).
Conventionally, a zoom lens unit with a large diameter, which includes, in order from an object side to the image side, a first lens group with positive refractive power, a second lens group with negative refractive power, a third lens group with negative refractive power, and a fourth lens group with positive refractive power having 7 to 8 lenses, is taught by Japanese Laid-Open Patent Applications No. 2006-030582 (Document 1), No. 1997(Hei09)-61715 (Document 2), No. 1997(Hei09)-61716 (Document 3), No. 1995(Hei07)-159693 (Document 4), No. 1998(Hei10)-054937 (Document 5), etc.
However, the zoom lens units disclosed in Documents 1 to 4 do not have a magnification ratio of approximately 20 times.
Further, even in Document 5, which might be considered as the closest reference to the embodiments of the present invention, aberration correction in the infrared light range is not considered at all.